This invention relates to cocatalysts or activators useful with metallocene catalysts, particularly with metallocene catalysts used to make olefin polymers.
Metallocene catalysts are well known, especially for polymerization of olefins. The term "metallocene catalysts" is used to mean organometallic compounds having a rare earth or transition metal in coordination with members of at least one five-membered carbon (preferably cyclopentadienyl) ring.
It is generally recognized that cocatalysts or activators are useful with metallocene catalysts, especially in polymerization of olefins. Known activating cocatalysts include e.g. polymeric or oligomeric alumoxanes, especially methylalumoxane, triisobutyl aluminum modified methylalumoxane, or diisobutylalumoxane; strong Lewis acids (the term "strong Lewis acid" as used herein is defined as trihydrocarbyl substituted Group 13 compounds, especially tri(hydrocarbyl)aluminum- or tri(hydrocarbyl)boron compounds and halogenated derivatives thereof, having from 1 to 10 carbons in each hydrocarbyl or halogenated hydrocarbyl group, more especially perfluorinated tri(aryl)boron compounds, and most especially tris(pentafluorophenyl)borane); amine, phosphine, aliphatic alcohol and mercaptan adducts of halogenated tri(C.sub.1-10 hydrocarbyl)boron compounds, especially such adducts of perfluorinated tri(aryl)boron compounds; nonpolymeric, ionic, compatible, noncoordinating, activating compounds (including the use of such compounds under oxidizing conditions); and combinations of the foregoing activating cocatalysts and techniques. Preferred species include tris(pentafluorophenyl)borane and the ionic activators containing the anion tetrakis (pentafluorophenyl)borate. These compounds are effective but require several pentafluorophenyl groups per molecule which are difficult to obtain or synthesize and are, therefore, quite expensive. Ionic activators react irreversibly with metallocenes and therefore may be preferred over the neutral strong Lewis acid activators, which may react reversibly with a metallocene. However, a commercial drawback of these ionic activators is their poor solubility in the polymerization medium, i.e., hydrocarbon solvents.
It would be desirable to have activators or cocatalysts which retain good solubility in hydrocarbon solvents and are preferably less likely to result in reversible reactions and/or which are less expensive, that is use less of the difficult to obtain pentafluorophenyl group.
Boratabenzenes are known and have been used as ligands but are not used as cocatalysts, activators or counterions for metallocene catalysts.
The boratabenzenes are anionic species which are boron containing analogues to benzene. They are previously known in the art having been described by G. Herberich, et al., in Organometallics, 14,1, 471-480 (1995). They may be prepared by reaction of stannocyclohexadiene and a borontrihalide followed by substitution with a hydrocarbyl group.